Workboard: improve the Create Task link to mention the Project slug

Summary:
This expands a specific link in a specific menu of Workboard Columns.

You may never notice this difference unless you like to open links in another tab.

If you go to a Workboard, and you open its context menu, and you hover your mouse on the
Create Task action, you see this URL or a similar one:

http://example.com/maniphest/task/edit/form/default/

After this change, you see this URL or a similar one:

http://example.com/maniphest/task/edit/form/default/?tags=test

You see that the PhutilURI class was used to add the Project slug to the 'tags' query param so
that users can still open the URL in a new tab and have the form prefilled with the Project Tag.

Closes T15147

Test Plan:
- visit a Workboard
- open the context menu of a Column (the pencil icon)
- see that the Create Task link has the Project slug is in the URL of its prefilled form

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15147

Differential Revision: https://we.phorge.it/D25068