Adds basic filtering to the Group Security Dashboard frontend

What does this MR do?

Adds the first iteration of filtering to the Group Security Dashboard

• Removes the tabbed interface
• Adds the filters module and relevant components
• Creates a filter for severity
• Creates a mocked filter for types
• Applies these filters to the GSD list
• Replaces the help popover with the gitlab-ui component and moves it into the filters

Screen_Recording_2018-12-17_at_18.52.26

Notes

• It currently only supports single select filters (e.g. one severity type). This is a UI limitation and will be dealt with in a future iteration.
• Similarly, project filtering and dismissal filtering will be handled in future iterations.

What are the relevant issue numbers?

#6240 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer