Mangle cells that look a little bit like formulas in CSV files

Summary:
Fixes T12800. See that task for discussion. When a cell in a CSV begins with "=", "+", "-", or "@", mangle the content to discourage Excel from executing it.

This is clumsy, but we support other formats (e.g., JSON) which preserve the data faithfully and you should probably be using JSON if you're going to do anything programmatic with it.

We could add two formats or a checkbox or a warning or something but cells with these symbols are fairly rare anyway.

Some possible exceptions I can think of are "user monograms" (but we don't export those right now) and "negative numbers" (but also no direct export today). We can add exceptions for those as they arise.

Test Plan: Exported a task named =cmd|'/C evil.exe'!A0, saw the title get mangled with "(!)" in front.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12800

Differential Revision: https://secure.phabricator.com/D18974