revise markdown so it can be on by default

#### Is your feature request related to a problem? Please describe.
Markdown defaults to off on android. As a frequent web/desktop user, I expected it to be on and was seriously confused (#3575) be it not being so.

I realize this was decided internally and it is thus on purpose. I want to make the argument that the decision should be revised long term.

#### Describe the solution you'd like.
Revise the md input such that it works intuitively. Educate users about its existence: This could be a killer feature or futuristic vision (since not many other mobile messaging apps provide (extensive) formatting). If you want to keep it default-off because other mobile apps don't have it either, a way to go could be:

"You have used special characters in your message that could be interpreted as formatting with markdown. \*preview without\* \*preview with md\* (link to more info)
do you want to enable md?"

"Yes / No / Don't ask again"

#### Describe alternatives you've considered.
- reminder to check settings/quick toggle on first launch
- save and sync settings

#### Additional context
parity with desktop