Add GitLab to update.py

[lauft]

Currently the update script for the tools page only scrapes GitHub for repositories connected to Timewarrior, tools on GitLab (and other hosting services) have to be added manually.

We should add GitLab to update.py to simplify this process and to include those Timewarrior tools available there.

The implementation should be done analogously to the process for GitHub.

[lauft]

There is overlap with #947, which wants to extract the tool scraping into a separate repository.