core: frontend: kraken: improve custom Extension installation process

[ES-Alexander]

Current behaviour

1. It is frequently the case that custom Extensions will be installed to test a development fix, in which case the Docker tag is the name of a repository branch. Since GitHub allows copying the name of the source branch from a pull request, it could serve as a convenient way to copy in the tag, but is currently hampered by the author name before the colon:

   which is not part of the Docker tag.

2. The save button is in the bottom left, which in inconsistent with the rest of the interface (and other common interface designs), and annoying to click on

Expected or desired behaviour

Ideally:

• [ ] pasting a pull-request source branch name into the Docker tag field would automatically remove anything up to and including a colon, if there is a colon in the pasted text
• [ ] the save button would be in the bottom right
• [ ] pressing Enter (or CTRL+Enter / CMD+Enter) would submit the form, instead of needing to manually click the button

Prerequisites

• [x] I have checked to make sure that a similar request has not already been filed or fixed.