Add push messaging (+ other assorted improvements)

[Rumperuu]

NB the Travis tests fail because it can't start the messaging service, but I think that will need someone with more Travis/Angular experience than me to fix.

This PR was originally intended to only add push notification functionality to the Web client. However, it has since expanded to into a bit of a monolith, which certainly isn't ideal and should be avoided in future, but I think I'm the only person working on this at the moment so it shouldn't cause any major issues.

This PR adds the following:

• an updated, clarified project readme, along with security and contributing guidelines;
• GitHub Issue templates;
• assorted cosmetic improvements and bugfixes;
• the ability for organisations to create topics;
• the ability for customers to subscribe to and unsubscribe from topics;
• the ability for organisations to push messages to customers; and
• push notification rendering via the Notifications API.

See also Pear-Trading/LocalSpend-Tracker#12 & Pear-Trading/Foodloop-Server#140

Closes #123 #125 #127 #132 #133 #140 #141