Tag colour coding in accountHistoryChart

[Derpius]

Currently switching from QWERTY to Colemak DHm and have been using tags to separate progress between layouts, and while the WPM difference makes QWERTY/Colemak obvious, the difference between Colemak DH and Colemak DHm is negligible, making them hard to compare in the graph.

The ability to set persistent colours for tags, along with displaying those colours in the account history graph would be helpful for any other tag use cases too, where the speed difference between them is minimal (e.g. swapping between different keyboards, or comparing speed on a touchscreen with a physical board).

If multiple coloured tags were added to the same data point, then the one highest in some sorting (likely alphabetical) would be used for each case, alternatively a user definable priority could be set, but I don't see a need for that.