Enable app links to get to specific units

[bsenyk]

Enable app links to get to specific units

[bsenyk]

Links should be in the following format:

General links: openmentoring://
Specific unit links: openmentoring://units/understand-4-digisec_4-secure-communications

[bsenyk]

Links between units should use the ionic routes for them.

For example: #/app/topics?unit=understand-4-digisec_4-secure-communications

[bsenyk]

The basic version of this is complete, however there are 2 known potential issues:

It doesn't check to see if the linked unit has been downloaded yet. There is already an issue for this -- #56

It is currently opening a new modal in front of the one that links to it. This means that if the user exits the linked unit, they will end up in the one they were in before they followed the link. I don't think this is ideal. @jmatsushita what do you think? If it should replace the current modal instead, we can create a separate ticket for that.