text: Title: SECURITY: bad regex pattern in 'gensim/corpora/wikicorpus.py' maybe cause 'ReDos' security problem.
  Body: #### Problem description
  
  i found two bad regex pattern in 'gensim/corpora/wikicorpus.py' 
  
  ```
  RE_P7 = re.compile(r'\n\[\[[iI]mage(.*?)(\|.*?)*\|(.*?)\]\]', re.UNICODE)
  """Keep description of images."""
  RE_P8 = re.compile(r'\n\[\[[fF]ile(.*?)(\|.*?)*\|(.*?)\]\]', re.UNICODE)
  """Keep description of files."""
  ```
  
  those pattern will cause 'ReDos' security problem,  proof of code like below
  
  ```
  import re
  RE_P8 = re.compile(r'\n\[\[[fF]ile(.*?)(\|.*?)*\|(.*?)\]\]', re.UNICODE)
  re.findall(RE_P8, "\n[[file"+"|a"*1000+"|]")
  ```
  
  run the above code, cpu utilization will be 100% in a very long period.
  
  
  more detail about 'ReDos' please see [owasp](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS).
  
  #### effect of this security problem 
  because i did not see anywhere use 'RE_P7' and 'RE_P8' pattern, and not familiar with gensim api, so i can not decide what's the effect of this security problem.