Remove trailing slash at the end of identity server

[NicolasBuquet]

Old Account data can have a trailing slash at the end of their Identity Server.
This can lead to unrecognized URL on the backend (like on 'invite to room') because the URL is then constructed with a double slash in its path.
This leads to backend error 500 for these calls.
So, fix this trailing slash as soon as we receive i

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request contains a changelog file in ./changelog.d. See https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes a sign off

[NicolasBuquet]

@pixlwave @giomfo
Can you have a quick look at this PR?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 19.91%. Comparing base (1c4b3f5) to head (3807109).
Report is 70 commits behind head on develop.

Files with missing lines	Patch %	Lines
MatrixSDK/MXSession.m	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1898      +/-   ##
===========================================
+ Coverage    19.21%   19.91%   +0.70%     
===========================================
  Files          617      564      -53     
  Lines        97349    79304   -18045     
  Branches     40896    33432    -7464     
===========================================
- Hits         18701    15795    -2906     
+ Misses       78102    63493   -14609     
+ Partials       546       16     -530     

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