AT Support Tables Embedding: Distinguish data not yet available from not applicable

[mcking65]

From review of aria-practices PR 2569, we need to provide more accurate copy in cells that do not contain data.

There are two reasons for a cell to not have data:

1. We have not yet tested the AT/Browser combination, e.g., JAWS/Firefox.
2. The AT/Browser combination does not exist, e.g., JAWS/Safari.

The copy in cells with no data should be:

• If the AT/Browser combination will be tested in the future, use the string "Data Not Yet Available"
• If the AT/Browser combination does not exist, use the string "Not Applicable".

There are other functions of the app that will need to reference data about valid AT/Browser combinations. So, we will need to determine how we want to represent these relationships in the data model.

[howard-e]

Work being done in #583 should address this

[howard-e]

Reopening since change is not yet included in production.

[ccanash]

This change is already in production. cc @mcking65

[mcking65]

fixed by #583