Replace "ins" tag with "u" tag for underline

https://github.com/jpuri/html-to-draftjs/blob/b91ba0be2cb8b4c5a635335641d499725ce181cc/src/library/processInlineTag.js#L6

"ins" tag corresponds to an insertion whereas "u" tag corresponds to underline. Moreover, "ins" tags are not correctly displayed as underline on many email platforms. Some of them change its colour, some others just don't underline anything. "u" is more appropriate here.