MR process: Maintenance improvements from recent work

[zepumph]

• [x] checkoutBranch() should have a third arg boolean for if it should call grunt output-js-project after checking it out.
• [ ] (see https://github.com/phetsims/perennial/issues/318) It would be awesome to rework getMaintenanceBranches to always get all possible branches, and then cache them for future use, either in .maintenance, or just in memory. Either way would be really nice.
• [x] A bunch of ======= printed above each new RC/production build, it would be much easier to see where we are at

@jonathanolson please let me know if you don't like these ideas. Or have other opinions.

[zepumph]

@jonathanolson can you review? I tested by using checkoutBranch on a sim that doesn't support output-js and one that does, and seeing that the new chipperSupportsOutputJSGruntTasks() works. See the side issue for the second topic.