Implement consistent "free preview" experience for users with partial access

Motivation

Open questions concerning non-patron users or users with partial access (sparked from #199):

In a list of items, should the "Free Preview" item be dynamic?
- I think it's important that free preview items be in the first screen of each series' item list.
- However, this could be done either by:
- having the most recent item in each series always available for preview; or
- pinning the free preview item to the top of the list (but this requires some styling to clearly show that it's not the newest item)
Should a user without access to a liturgy be able to list the items in the liturgy?
- I lean towards yes, so that we can show them exactly what they would get if they pledged.
- However, we probably want the shading to be consistently associated with tap behavior - i.e. tapping on a dimmed (inaccessible) item always navigates to the Patreon screen.

Let's discuss these questions in the comments on this issue, and then track their implementation here as well.

[ ] Resolve the design questions in the comments below
[ ] Implement the "Free Preview" approach decided upon
[ ] Implement consistent and coherent series labeling for users with partial access