Rename `Invoices` tab to `Activity` tab

Continued effort to try to consolidate all of the Invoices/Recent TX/etc.. views under a single area, if possible (more discussion will take place around how best to deal with this).

It makes sense to rename this section to start thinking in these terms.

No file or test is named. Start by locating the navigation entry and view that currently display the Invoices tab, then check related Recent TX or invoice views for naming consistency. Done means the section is labeled Activity without disrupting its existing navigation or content.

Written by the indexing model from the issue text.