Strip all variation selectors on emoji
...when inserting into or looking up in the unicode to emoji map.
This broke with emojibase 4.2.0 which changed the type of a whole
load of emojis to 'text' when previously they were 'emoji'. This
caused them to get the 'text' variant of the unicode string which
has the text variation selector (15) appended instead of the emoji
variation selector (16). We were only stripping the emoji selector,
so upgrading to 4.2.0 caused riot to fail to find the heart in the
unicode map, which therefore prevented the app from starting.
