text: Title: Add setting to disable the broken website alert
  Body: ### Describe the issue
  
  We're getting alerts on pretty much all components in https://l10n.xwiki.org related to Broken project website, with the following error:
  > (403 Client Error: Forbidden for url: https://www.xwiki.org/xwiki/bin/view/Main/WebHome)
  
  I think the issue is that our sysadmins configured www.xwiki.org to prevent some bots to access it and so it's blocking also Weblate checks. Now for us this check is completely useless as we control the components we create, so I set `WEBSITE_REQUIRED=False` in settings and I manually triggered the tasks `repository_alerts` and `component_alerts`, but I'm still having the alert displayed. And I don't find a way to globally dismiss it. 
  
  ### I already tried
  
  - [X] I've read and searched [the documentation](https://docs.weblate.org/).
  - [X] I've searched for similar filed issues in this repository.
  
  ### Steps to reproduce the behavior
  
  1. Create a project and indicate the URL https://www.xwiki.org
  2. See that an alert is created for broken project website
  3. Check that the website is actually available
  
  ### Expected behavior
  
  Website availability should be checked with some specific documented headers so that it's possible to filter them out to not consider them as bots. 
  Also it should be possible to entirely ignore those alerts with some configuration, and if possible to be able to bulk dismiss alerts.
  
  ### Screenshots
  
  ![image](https://github.com/user-attachments/assets/fe30354d-b1f4-4a7e-8d1e-dbe12de9ebb6)
  
  
  ### Exception traceback
  
  _No response_
  
  ### How do you run Weblate?
  
  PyPI module
  
  ### Weblate versions
  
  Weblate: 5.9.2
  Django: 5.1.4
  siphashc: 2.3
  translate-toolkit: 3.14.5
  lxml: 5.3.0
  pillow: 11.1.0
  nh3: 0.2.15
  python-dateutil: 2.8.2
  social-auth-core: 4.5.1
  social-auth-app-django: 5.4.2
  django-crispy-forms: 2.3
  oauthlib: 3.2.2
  django-compressor: 4.4
  djangorestframework: 3.15.2
  django-filter: 24.3
  django-appconf: 1.0.6
  user-agents: 2.2.0
  filelock: 3.16.1
  RapidFuzz: 3.11.0
  openpyxl: 3.1.2
  celery: 5.4.0
  django-celery-beat: 2.7.0
  kombu: 5.3.4
  translation-finder: 2.19
  weblate-language-data: 2024.16
  html2text: 2024.2.26
  pycairo: 1.25.1
  PyGObject: 3.46.0
  diff-match-patch: 20241021
  requests: 2.32.3
  django-redis: 5.4.0
  hiredis: 2.2.3
  sentry-sdk: 2.19.2
  Cython: 3.0.11
  mistletoe: 1.4.0
  GitPython: 3.1.40
  borgbackup: 1.2.7
  pyparsing: 3.1.1
  ahocorasick_rs: 0.22.1
  python-redis-lock: 4.0.0
  charset-normalizer: 3.3.2
  cyrtranslit: 1.1.1
  drf-spectacular: 0.28.0
  Python: 3.11.2
  Git: 2.39.5
  psycopg: 3.1.17
  psycopg-binary: 3.1.17
  phply: 1.2.6
  ruamel.yaml: 0.17.40
  tesserocr: 2.6.2
  boto3: 1.28.85
  aeidon: 1.15
  iniparse: 0.5
  google-cloud-translate: 3.19.0
  openai: 1.59.7
  Mercurial: 6.9
  git-review: 2.4.0
  PostgreSQL server: 15.10
  Database backends: django.db.backends.postgresql
  PostgreSQL implementation: psycopg3 (binary)
  Cache backends: default:RedisCache, avatar:FileBasedCache
  Email setup: django.core.mail.backends.smtp.EmailBackend: localhost
  OS encoding: filesystem=utf-8, default=utf-8
  Celery: redis://localhost:6379, redis://localhost:6379, regular
  Platform: Linux 6.1.0-17-amd64 (x86_64)
  
  ### Weblate deploy checks
  
  _No response_
  
  ### Additional context
  
  _No response_