docs/testplan: add admin UI testplan

[littleskunk]

What: A testplan for the new admin UI aka backoffice.

Why: We had some issues with the old admin UI. It overrides the placement constraint in some situations. Luckily we noticed it and corrected the placement constraint. To make sure this doesn't happen again we should spend a bit more time on tests.

Please describe the tests:

• Test 1:
• Test 2:

Please describe the performance impact:

Code Review Checklist (to be filled out by reviewer)

• NEW: Are there any Satellite database migrations? Are they forwards and backwards compatible?
• Does the PR describe what changes are being made?
• Does the PR describe why the changes are being made?
• Does the code follow our style guide?
• Does the code follow our testing guide?
• Is the PR appropriately sized? (If it could be broken into smaller PRs it should be)
• Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
• Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
• Does any documentation need updating?
• Do the database access patterns make sense?

[ifraixedes]

It looks good to me, I dropped a couple of comments for a functionality that was not planed for v1.

I would suggest @stoweandi to review it too.

@littleskunk if you think that we should have a meeting, let's schedule one.