Remove ember-cli-flash dependency

[jelhan]

Currently both ember-cli-flash and <BsAlert> component of Ember Bootstrap are used to render warning and error messages. We should drop ember-cli-flash dependency to reduce technical complexity.

ember-cli-flash is used for two features:

1. Inform the user that the poll link has been copied successfully.
2. Show an error when saving the poll failed.

The first use case is better solved with another pattern. Instead of rendering a success message somewhere else, the copy button should report success directly.

The second use case is well served by <BsAlert>.

[jelhan]

The second use case is well served by <BsAlert>.

Error is reported using a modal if saving a participation fails. For consistency we should use a modal here as well.