Message/alert directive; style-docs page

There are three different things in this PR:

1. Create new directive for flash messages. Now, `<flash-messages></flash-messages>` will show all the flash messages in that spot (if any).
2. Added alert directive that generates the same code for using bootstrap alerts. This will be styled later.
3. Added a style docs page (http://localhost:8000/#/style-docs) which currently contains an example on how to use the alerts directive. Future directives and client styling stuff will go here.