Add the `/solves` command to specify that a PR solves an issue. For example `/solves SKARA-155` or `/solves 155` would both add the issue under the "Issue" header (should probably be renamed "Issues") and also close the issue once the PR is pushed.

For bonus points, if the PR does not have a correct title of the form "12345: This is a bug" and a user writes `/solves SKARA-79`, also update the title to "79: Add foo to bar".