Add logic for loading-spinner on forms

[petesfrench]

Done

• Copies the code from u.com to the vanilla loading spinner on successful form submission

QA

• fill in a modal form check that the spinner is attached on form submission
• do the same for a static form

Issue / Card

Fixes https://warthogs.atlassian.net/browse/WD-8603

[webteam-app]

Demo starting at https://canonical-com-1207.demos.haus

[lizzochek]

Except the comment from Carlos looks good to me

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.45%. Comparing base (472e0e8) to head (4a76f4c). Report is 102 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1207 +/- ## ========================================== + Coverage 66.36% 71.45% +5.09% ========================================== Files 10 12 +2 Lines 886 1023 +137 ========================================== + Hits 588 731 +143 + Misses 298 292 -6 ``` | [Flag](https://app.codecov.io/gh/canonical/canonical.com/pull/1207/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | Coverage Δ | | |---|---|---| | [python](https://app.codecov.io/gh/canonical/canonical.com/pull/1207/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | `71.45% <ø> (+5.09%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical#carryforward-flags-in-the-pull-request-comment) to find out more.

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