Added every alert class in MessageProvider

- added default message (grey border)
- added info message (blue background)
- added warning message (yellow background)

- added link to alerts on bootstrap
id for search box

Search box now has id „keyword“ so it can be targeted with JavaScript
Fixed open else & changed 0 results message

- closed open else (search with 0 results) and added comment to clarify
what } on line 215 does
- set „No songs found“ message to warning not dismissable message so it
is more eye-catching
- added link to search box in „No songs found“ message to directly
focus search box
Display filter & default alert

- some styles for display filter
- border for default alert