ManiphestReportController: Separate legacy and synthetic data handling

Summary:
Due to code additions in rPcb957f8d and rPadbd7d4f required due to rPd321cc81, the code intertwines handling legacy data with handling/creating modern data.
Make things more understandable by clearly separating between both (handle one after the other) and by renaming some variables for clarity, so it will become slightly easier in the future to investigate this bottleneck (it is the only code querying the ManiphestTransaction table, leading to timeouts in large Phorge installations).

Also add a specific reference to the corresponding code change in a code comment, instead of a vague "late 2017".
Also, don't use the variable name $table for two different things (database vs AphrontTableView) in the same function.

Test Plan: Carefully read the code. Optionally, play with http://phorge.localhost/maniphest/report/burn/ with and without setting a project filter having tasks created in the codebase before 2017-11-22, and compare that the output is still the same.

Reviewers: O1 Blessed Committers, 20after4

Reviewed By: O1 Blessed Committers, 20after4

Subscribers: 20after4, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25828