Support removing trailing whitespaces in Web IDE

Problem to solve

When using the Web IDE to edit certain files, for example, GitLab documentation pages, the file will often end up with trailing white space characters. GitLab's docs lint job will fail when this happens, and it's difficult using the Web IDE to remove these. Instead, I find I have to pull the branch and save it in my local editor which is set to strip trailing whitespace characters before pushing the file back up.

Proposal

The Web IDE should have a setting to allow users to strip trailing whitespace characters automatically from the file before saving.

Projects and Groups should allow Admins to specify what this default should be for this setting.

Optionally, admins should be able to have the setting apply as the default to only some filetypes and not others.