feat: make user session timeout configurable by flags

[fritx]

ref: #2753

Description

🚨 Before submitting your PR, please read community, and indicate which issues (in any of the repos) are either fixed or closed by this PR. See GitHub Help: Closing issues using keywords.

• DO make sure you are requesting to pull a topic/feature/bugfix branch (right side). Don't request your master!
• DO make sure you are making a pull request against the master branch (left side). Also you should start your branch off our master.
• DO make sure that File Browser can be successfully built. See builds and development.
• DO make sure that related issues are opened in other repositories. I.e., the frontend, caddy plugins or the web page need to be updated accordingly.
• AVOID breaking the continuous integration build.

Further comments

[hodgesp]

Tested in a local copy, need to update docs I think. For what my review is worth.

[fritx]

Tested in a local copy, need to update docs I think. For what my review is worth.

Thanks for you help and test. Which docs need to be updated? I've found that the other flags are not mentioned neither.

[hodgesp]

Tested in a local copy, need to update docs I think. For what my review is worth.

Thanks for you help and test. Which docs need to be updated? I've found that the other flags are not mentioned neither.

I do not know, just a user myself. Maybe a second read-me with the flags and runtime options but most documentation is over on filebrowser.org so there may be a separate repo for that docs site content or use https://github.com/filebrowser/docs