Avoids the use of HTMX on milestone assignment within a New Issue form.
The New Issue form doesn't have an issue ID to send to a milestone change URL,
which the backend expects in order to construct a proper reply. The frontend
template was also not built to use the new HTMX response. This resulted in a
backend error and a large warning whenever anyone tried to set a milestone
from within the New Issue form (new pull requests were also affected), rather
than from a View Issue page.
This introduces a new parameter into the `issue/milestone/select_menu`
template, "NewIssuePage".
When unset, the template produces the same results as before. Selection uses
`hx-post` to notify the server immediately, using the updated htmx fragment
from the reply.
When set to a truthy value, the old style of form is used. Selection uses
`data-id` and `data-href` to update the selected milestone locally, via
`selectItem` in `repo-legacy.js`, recreating the selected element and updating
the hidden form value.
Fixes#5176.
- Currently if you want to update the milestone of an issue or pull
request, your whole page will be reloaded to reflect the newly set
milestone. This is quite unecessary, as only the milestone text is
updated and a new timeline event is added.
- This patch converts the milestone section in the issue/pull request
sidebar to use HTMX, so it becomes a progressive element and avoids
reloading the whole page to update the milestone.
- The update of the milestone section itself is quite straightforward
and nothing special is happening. To support adding new timeline events,
a new element `#insert-timeline` is conviently placed after the last
timeline event, which can be used with
[`hx-swap-oob`](https://htmx.org/attributes/hx-swap-oob/) to position
new timeline events before that element.
- Adds E2E test.
