[Scenario] Remove repositories from top-level taxonomy

Per nasa/MCT-Plugins#134, User Tags, Mission Tags, etc.
should no longer be bootstrap (top-level) components,
but should be nested instead.
[Scenario] Add Mission to top-level taxonomy
[Scenario] Remove bootstrap capability from Repository

RepositoryComponents are no longer top-level
components per revisions to taxonomy in
nasa/MCT-Plugins#134, so they no longer need to
or should implement the Bootstrap capability
(which controls positioning of top-level
components.)
[Scenario] Add templates to mission

Add Mission Templates to the top-level Mission
taxonomy element; Mission Templates in turn
contains Mission Tags and Mission Activity
Types. Includes workarounds in case of existing
components.
[Scenario] Refactor taxonomy initialization

Move taxonomy initialization for the scenario
distribution into its own class, to reduce
amount of code dedicated to this concern in
the plugin provider.
[Scenario] Add My Templates to My Sandbox
[Scenario] Disallow removal of My Templates
[Scenario] Switch OK/Cancel in tag selection dialog

Order of OK and Cancel was reversed relative
to other dialogs in MCT; corrected.

This is a quick-fix for error in implementation of
nasa/MCT-Plugins#129
Merge pull request #146 from VWoeltjen/mctPlugins134

[Scenario] Revise top-level taxonomy