Improve initial instruction on web.threema.ch

[Simon Heimberg]

When navigating to web.threema.ch, the instruction how to proceed is very short.

Currently the message is:
Scan this QR code with your Threema app
But it is not obvious, where in the app you do scanning.

I suggest something like:
Scan this QR code with your Threema app. To do this, start the app and tap on menu (icon "...") (Android) or Settings (icon "gear wheel") on iOS, then select "Threema Web" and follow the instructions there.

This is a small change requiring only adaption of (translated) text.
The translation label is PLEASE_SCAN in

threema-web/public/i18n/en.json

Line 8 in 1656f80

 "PLEASE_SCAN": "Scan this QR code with your Threema app", 

Maybe this is adapted with this translation label, maybe a 2nd label introduced (like PLEASE_SCAN_INSTR).

[Simon Heimberg]

The translation label is used in this place.

threema-web/src/partials/welcome.html

Line 9 in 7df69c1

 <p class="instructions" translate>welcome.PLEASE_SCAN</p> 

Adding a second label (like PLEASE_SCAN_INSTR) gives options for formatting. It could be above or below the QR code, or as a tool tip (title attribute), or ...