Allow permanent static UIDs for webserver accounts

Summary:
Currently, webserver-alkane accounts to run PHP applications
doesn't include a specific UID in the configuration.

We would like to avoid several issues with dynamic UIDs:

• new accounts created on the fly will eat a position
• different accounts on different servers share the same dynamic UID

Plan is to first assign UIDs for future websites, starting with ViMbAdmin,
then to assign UIDs to existing ones. The parameter needs to be optional
until that backfill assignment is done.

Ref T1932
Ref T1971

Test Plan: Apply future states for ViMbAdmin

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1932, T1971

Differential Revision: https://devcentral.nasqueron.org/D3340