Improve commit panel in Web IDE

The commit panel switches entirely between multiple states which is disorienting. We should try to keep the states similar so that the change in state is less jarring.

Proposal

• Always show the heading Staged
• Always show commit area at the bottom

Design

• Replace staged/unstaged changes sections with empty state illustration (keep commit window visible)

• Replace commit section with empty state illustration (have it disappear on a timer of 10 seconds)

We also need a way to show:

• commit on new branch (copy Your changes have been committed. Commit 33ee44rr on branch X)
• commit on new branch with merge request (copy Your changes have been committed. Commit 33ee44rr on branch X, along with merge request !1234)
• failure (Show a red cross illustration for if there is a failure) (copy Due to an error your changes have __not__ been committed)