Offer opening unsupported filetypes with external app

[kekkyojin]

PR Checklist

Please check all that apply to this PR using "x":

• I have checked that this PR is not a duplicate of an existing PR (open, closed or merged)
• I have checked that this PR does not introduce a breaking change
• This PR introduces breaking changes and I have provided a detailed explanation below

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting)
• Refactoring (no functional changes)
• Documentation changes
• Other - Please describe:

Fixes

Issue Number: #419

What is the current behavior?

If LBRY doesn't include the capacity of showing the content on the claim, user should download file and search for it on its device and then open it

What is the new behavior?

User is shown a notice about LBRY inability to show content and a button which allows it to choose a compatible app when clicked.

Other information

This PR adds a new string and reuses other one.

In order to test it, reviewer could search for "PDF" on LBRY Android and look for one without a time on the snapshot. I used the ones with "UOC" on their title. "UOC PDF" could be a better approach for the search text.

[akinwale]

This is great! Thanks. I've been wanting to tackle this for a while, but I never got around to it yet.