Investigate removal of DB projects feature

[flodolo]

We should check how much work it's required to replace existing projects that are using this, and eventually remove the feature completely.

This would reduce maintenance and simplify sync code.

[mathjazz]

There are two DB projects currently in use:

• Tutorial
• Terminology

Two other DB projects have been deactivated:

• Engagement Firefox Rocket Keyword ASO
• Engagement HeartBeat Apr2018

This is the PR that introduced the DB projects. 99% of the code is the Admin interface, the rest is two minor changes:

• Add Project.data_source field
• Exclude DB projects from sync

We don't need the Admin interface for the currently used projects, so we can drop it.

We can also drop Project.data_source and identify DB projects as projects without repositories when excluding them from sync.

[mathjazz]

See also #2126.