Nested <sub> tags not handled properly when switching between modes

[b-kelly]

https://meta.stackexchange.com/a/360181/395497

In the current editor using nested <sub></sub> html is supported, and can achieve an extra small fontsize. For example: <sub><sub>Extra Small </sub></sub>

When writing this in the Markdown version of the RT editor, and then switching over to RT the first <sub> gets trimmed, the last </sub> doesn't: <sub>Extra Small</sub></sub>

Either nested <sub> statements should be supported, or the last closing argument should also be trimmed.

[yellis]

See #162 - pending fix upstream, moving to beta 3