When you visit the Members page of a Parent Subproject,
some actions are greyed out by design. And, if you click on
them, nothing really useful happens to continue with what
you want to do.

For example, see this page and click on "Join Project":

https://we.phorge.it/project/members/1/

After this change, the interface at least provides a link to
proceed to the next step of what you wanted to do - that is -
allowing to quickly list direct Subprojects and do something
more meaningful than that.

A/B for the "Join Project" view:

Before	After

A/B for the "Add Members" view:

Before	After

Just to be nice, this change adds some inline documentation
on some methods of AphrontDialogView used here.

Closes T15258

• Create a Project (A) with a Subproject (B)
• Visit Members page of (A)
• Click on Join Project and click on the Blue Button
• Click on the Add Member and click on Blue Button
• in both cases it should get you to Subprojects list