Reduce code duplication surrounding the :welcome module

[Cliabhach]

The issue

As it stands at v1.0, there are some significant quantities of code duplication between

• WelcomeTourCellViewModel and AllToursCellViewModel
• WelcomeEventCellViewModel and AllEventsCellViewModel
• WelcomeExhibitionCellViewModel and AllExhibitionsCellViewModel
• AllEventsFragment, AllToursFragment, and AllExhibitionsFragment
• the layout files used by above classes
• the navigation paradigms used by above classes
• the RecyclerView adapters used by the above classes.

This causes no small amount of overhead when editing related visual elements (c.f. PRs #220, #230, #240, #260, #268, #289, etc).

Where it came from

I harbor no ill will towards the existing structure of these elements. Earlier understandings of the desired screens had less commonality and the development team thus took an apprehensive stance towards consolidation. At this stage in the application development process, though, we can be assured that the degree of deviation is much less than then-anticipated.

Resolution plan

By combining the related files into one module, it should be easier to detect inadvertent inconsistencies and share logic to the greatest extent permissible. The new module's name should represent the shared goal of the files aligned with AllEvents, AllExhibitions and AllTours. Name suggestions are welcome, of course, but I will start work with the working title :content_listing for now.