Migrate all possible SVN URLs to GitLab

[alexpearce]

@saschastahl has already changed some SVN URLs to corresponding GitLab paths in #212. We should do this for all SVN URLs in the lessons.

There may be some projects that we can't do this for yet, such as DBASE and DecFiles, but I can't remember if we link to those anywhere.

[alexpearce]

One example (can solve this with separate PRs): the link in the tuple tools and branches lesson points to the SVN repo for the list of tuple tools.