Update checklist page

[kaitj]

Made a few quick changes to the checklist page stemming from feedback during previous tool review session - should help with the review process after checklist has been submitted.

1. Moved the "export" and "import" buttons into separate modals. All the functionality remains same from there + a few additional buttons (see points 2 + 3). For both modals, an explicit "close" button is added, although it can also be closed just by clicking outside of the modal.
   

2. For exporting, "json" and "csv" options remain with an additional "copy" option, which copies the checklist to the clipboard. This should enable easier process of updating an existing checklist issue during the review process.
   

3. For importing, the option to import from json is still available. However, rather than immediately rendering the checklist, it first outputs it into the textarea. Alternatively, a user could also manually enter the checklist (or paste in an existing one), which should also make reviewing easier (i.e. copy json from issue and paste into text area here). One the upload button is pressed, the checklist is rendered.
   (note: comment about importing from URL has been dropped from the page.)
   

Played around with the idea of a rendering from a URL and ultimately could still be something to be done. However, there was also discussion of adding evidence / comment free text fields, complicating what would already be a long url query parameter.

Here's a quick video demoing what this currently looks like:

nmind_checklist_demo.mp4

[kaitj]

@shnizzedy @effigies - would either of you have time to give this a quick look? Hoping to maybe get this in to make the review process easier.

[shnizzedy]

I think this looks great.

A separate issue (this is the case despite this PR, just easier to notice after these changes), but I think worth handling:

If a user imports JSON, the relevant key-values are rendered, booleans are handled as truthy, and irrelevant key-values seem to be ignored (that all seems good to me). Then if/when the checklist is exported, the original non-validated inputs are included (extra keys, non-boolean truthy values). We might want to drop extra keys
and coerce types, either on import or before export. Or we might want to actually validate and reject JSONs that don't match the schema.