[mobile] Allow selecting text in messages

In the mobile client, I sometimes want to copy or share part of a message. Currently the only way to long tap the message to open the context menu, copy the message, paste it to the composer or another app, find the text you wanted to select if it's a long message, select it and finally copy or share it.

Allow selecting part of a message directly. There are multiple possible designs for this, I really like Telegram's - example (also attached, with copy or share instead of quote), but it doesn't directly translate to a Fluxer's UI. Another idea I've had is tap-then-hold (i.e. double tap without releasing the tap) but it would have really bad discoverability.