Add gitlab login support

[mirguest]

Release Note:

• Allow user to use the GitLab OAuth to login the HedgeDoc

The detail implementation:

• A helper script login-hedgedoc-via-gitlab.sh is used to wrapper the process to login via gitlab.
• This helper script consists following part:
  • Step 1: access the callback of HedgeDoc "auth/gitlab". From the response header, the location will be retrieved.
  • Step 2: access the callback of GitLab. From the response body, the login url will be retrieved.
  • Step 3: login the GitLab. It consists two steps:
    • Step 3.1: access the login url, and get the authenticity_token.
    • Step 3.2: access the real login url and the username/password wil be posted here.
  • Step 4: access the callback of GitLab again. Then the redirection URL(HedgeDoc) will be got here.
  • Step 5: access the HedgeDoc.

Note:

• If accessing multiple HedgeDoc servers, make sure the cookie files are different.

[drupol]

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