Allow changing database name

[DavidMStraub]

Currently, the database cannot be renamed via Web API.

Note: the database name is stored in name.txt in the tree directory.

Challenges: in single-tree setups, we still use TREE, the name, to pick the right tree, so it doesn't make sense to change the name at runtime.

Options:

1. Disallow renaming tree in single-tree setup
2. Deprecate TREE in favour of something like TREE_ID and block the new feature until then - but this would be a breaking change for all users, not great
3. Introduce a new TREE_ID option and only allow renaming tree if it is set - still requires all users to change their config
4. Allow using a tree ID for TREE and detect whether it's a name or an ID, allow renaming only if it's an ID - complicated
5. Remove single-tree setup and TREE config option???

Blocks: gramps-project/gramps-web#527