Ignore VS16 char in RTE

[pafcu]

Fixes element-hq/element-web#5049

[lukebarnard1]

It seems that @richvdh has been in this area recently (joypixels/emojione#320) and apparently the issue should have been resolved in emojione 2.2.7 (the same version that we depend on).

Also, I wonder if we should be worrying about VS 15 as well:
https://en.wikipedia.org/wiki/Variant_form_(Unicode)#Variation_Selectors_block

[pafcu]

@lukebarnard1 So it seems that there is either still a bug in emojione, or an old version of emojione is being used for some reason. Because current develop does not seem to work correctly (test e.g. by copy-pasting symbols from https://en.wikipedia.org/wiki/Dingbat#Emoji)

VS15 should definitely also be handled, but that is a separate issue. Similarly the per-symbol default presentation is ignored and everything seems to be treated as emoji-by-default (which, however, may be an acceptable simplification in order to reduce user confusion even if it breaks the unicode spec).

[pafcu]

Finally it seems like the fix in Emojione is not the one we needed. There are no longer the duplicate entries (as reported in joypixels/emojione#320) but the VS16 variant also does not map to the same character as the variant-less version, but to the one with the variant appended. I think this is actually "correct" in the sense that for eg. U+2709 (the envelop) the default presentation is text, so U+2709 U+FE0F (emoji version) should look different / be separate from the variantless version.

Therefore I think that Emojione is not strictly wrong here, and that the proposed change here is the correct way to go without making some fairly large changes to how emojies are handled.

[lukebarnard1]

WFM, worst case this will affect the display of other emoji.

LGTM