Add an 'allow rebasing' toggle on per-branch basis

[krlvi]

The app has a project/repo wide setting allow force pushing, which defaults to true. This config is responsible for the following:

• If the app should use a merge commit or perform a rebase for the branches when the workspace is being updated with new changes from master, after a branch has been pushed to a remote.
• If the app should use a merge commit or perform a rebase for a branch when the user clicks "Integrate upstream" (to add new commits locally from the remote upstream, i.e. when collaborating with someone else)
• If the app should allow amending / reordering of commits that have already been pushed

In the scenarios above, a subsequent push will need to be a "force push", which may be undesirable in some cases, hence the ability to disable that behavior with the allow force pushing config.

This PR adds the ability to control this behavior on a per-branch basis. For example, if one branch is used for collaborating with another developer, it may be desirable to toggle this off for that specific branch only, while keeping it enabled for other branches (since rebasing offers more flexibility in the workflow).

This can be toggled with the "Allow rebasing" toggle on the branch lane.

The default value will follow the value of the project/repo-wide allow force pushing (which is enabled by default).