Full support for text styling

[martgil]

          For now FlowCrypt supports only plain text messages, without custom fonts, (experimental support for rich text is currently available on firefox, but I think it supports only text styling, but not custom fonts) so it's not possible to apply custom fonts to signatures.

We'll come back to this issue when extension will have full support for text styling and custom fonts.

Originally posted by @sosnovsky in #5362 (comment)

[martgil]

I have moved this to a separate GitHub issue as we have received a feature suggestion to add support for text styling and formatting availability (rich text mode) in Google Chrome. This feature is currently available on the Firefox version of the extension as an experimental feature.

Reference:
https://mail.google.com/mail/u/[email protected]/#inbox/FMfcgzGtwqPTnHBpSzBMpHbJtPxXjlSC
https://mail.google.com/mail/u/[email protected]/#inbox/FMfcgzGwHLhdlFLmVsdszBGwFjsKZLSG