Better success/error messages for the settings page and user insights ratings

Ref #335 (closed)

Replaces native js alert() with the bootstrap alert component.

This can be tested by rating insights, or saving settings on the settings page (save empty email/slack settings for an error message, language/detective settings for success).

Messages disappear after 5 seconds. They stack on top of each others if there are several in the same 5-seconds interval, and disappear individually.

We can discuss about the design: e.g. more flashy colors / css animation to draw more attention to them?