Change icon for the date export on the lesson admin page

Problem/Motivation

On the lesson admin page, for course offerings, there are two buttons marked "CSV". One is a CSV export of the lessons and their content, one is an export of the schedule only. There are alt texts for the buttons, but it is not 100% clear that they do different things.

Proposed resolution

Replace the schedule export with a TXT button, either by changing export plugin to txt, or by altering the view with a hook.

Remaining tasks

Do the stuff.

User interface changes

(as described above)

API changes

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