Subscribe to individual merge proposals

[Andrea Corbellini]

To receive notifications about merge proposals, you must subscribe to the source branch, the target branch or both, setting "Code review level" to a desired value. This causes several problems:

1. You receive notifications about all the merge proposals related to the branch, and not just the MPs you are interested in.
2. You cannot make the subscription directly from the MP page, but you must do it from the branch page.
3. If you are subscribed just to the target branch, you don't receive notifications of new commits.
4. You cannot modify the verbosity of individual merge proposals (because all merge proposals related to a branch will have the same "Code review level").