Update IRelation to support multiple PagePaths, so relations can be reused
Remove hardcoded `/umbraco/` reference
Add Helper to generate links/icons
Misc tweaks/polish to relations, add new
Misc tidying
Fix hardcoded field reference in XPath
Refactor to separate relations and the objects they relate to

* Add concept of UmbracoObject to store info about each related entity
* Refactor individual relations to have their Table generated by the UmbracoObject, so it can be reused across multipe relations (ie same visualization for Data Type -> Document Type and Template -> Document Type), use new objects for from/to
* Update toolbar logic to determine pages based on the UmbracoObject instead of the PagePath of the individual relation
* Update usages page to find available relations based on the UmbracoObject
* Add list of available UmbracoObjects, method to determine whether they should show based on the page path
Add tooltip to content nodes showing the path
Add name of object/page to modal title

* Would be nice to show the name of the item we're viewing too, ie Datatype Usages: DAMP.  Just need to add a NameById lookup on each object type.
Minor fixes
Add Description to describe what each relation is checking for
Tweak toolbar icon
Show friendly type name on tabs
Housekeeping