Enhance uploaded file status text read by screenreaders

[ggeisler]

Related to PR #853. In that PR I added "Type of file: " to the existing visually-hidden uploaded file status text:

<span class="visually-hidden">Type of file: <%= file.pod_metadata_status %></span>

That appears in the first column of the Upload activity (Activity page) and Uploaded files (Provider home) tables.

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However, the file.pod_metadata_status value is not informative enough for a screenreader user. The current values are one-word values like "unknown" and "success". To help the screenreader user who doesn't have visual access to the file status icon legend above the table, we should use the legend values for file.pod_metadata_status.

In other words, for a file that currently has a rendered value of success it should instead be Valid MARC (adds, updates, or deletes) so the screenreader user hears "Type of file: Valid MARC (adds, updates, or deletes)".