Reduce redundant downloads

[Brian Lewis]

Files stored in the filedb are immutable - the file assigned to an id is never changed. So we can use cache control in the browser to reduce the number of times files are downloaded.

Three separate cases to address:

1. \library calls into the filedb. Note that because the bearer token is in the query string, we can only cache for the duration of the current login session (8 hours). This case will affect download of Kiribati survey form pdfs, (which are typically 5Mb), and census Excel workbooks.
2. Logo. Logo is fixed and should survive from session to session. In Kiribati, this file is 189K, so it slows startup and popup logins.
3. Editable grids. The code fragments used in editable grid are supplied by the mvc ComponentCustomisationController. ui-grid forces a read of these every time a row is rendered, which means they are hit dozen of times when you scroll the grid. A long-standing annoyance!

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[Brian Lewis]

task(performance): reduce repeat downloads

Resolves #1221

→ <<cset 861261662a6b>>