Workboard: Milestone Name easily editable (instead of surfing 3 pages)

Summary:
After this change, a new input field "Milestone Name" appears in the "Edit" menu of a Milestone:

| Before | After |
|-----------|-----------|
| {F314008} | {F314005} |

So you can quickly change the name of your Milestones, from a Workboard.

Before this change, from a Workboard, this was the way to rename a Milestone:

1. click on the Milestone name (yes, that is a link)
2. click on Manage
3. click on Edit Details
4. rename
5. Save
6. Manually visit again the Project's Workboard

After this change, from a Workboard, you just need to:

1. click on Milestone > Edit
2. click on Edit Column
3. rename
4. Save

Example usage:

{F314015}

This does not change the level of permissions needed: if you have not enough
permissions to see or edit a Milestone, you cannot access this feature indeed.

In short, this is just a frontend change, keeping current policies as-is.

Closes T15143

Test Plan:
Create a Project or use an existing editable one.

Create a Milestone called "Test Milestone". You can create Milestones visiting the Project's menu {nav icon=sitemap,name=Subprojects > icon=plus,name=Create next milestone}

Visit the Project's Workboard. Find the column "Test Milestone". Click the Edit button on a Milestone, and:

- try to save another name: it must work
- try to save an empty name: nice error message shown
- try to save both the score points and the name: it must work
- try to save "FOO" as Points: you still see the error message

Also:

- do the same for the Backlog column: it still works (name still allowed to be empty)
- do the same for a "normal" Column (not the Backlog): it still work (name still __not__ allowed to be empty)

Reviewers: O1 Blessed Committers, Cigaryno, 20after4, waldyrious

Reviewed By: O1 Blessed Committers, Cigaryno, 20after4, waldyrious

Subscribers: waldyrious, brennen, aklapper, 20after4, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15143

Differential Revision: https://we.phorge.it/D25066