General Minor Enhancements Part 3

Continuing from #65

- [ ] in the doPost() function, add better feedback when something is 'loading' (like a global loading symbol or loading bar which runs across the screen)
- [ ] Manually added mod option downloads are always added *after* the normal options. Therefore their priority levels can't be in-between the existing downloads, only can come after them. Not sure if this is the desired behavior, or if there is even a case where you would want this. But the behavior is confusing at the moment as in the JSON it's called "priority' for both priorities, even though they are different priorities.
- [ ] Add option to download only (stop after downloads complete)
- [ ] Add "open downloads folder" / "open installer folder" button
- [ ] Add free disk space check (Not implemented on mac, but otherwise DONE)
- [ ] Fix duplicate mod size (probably remove top mod size warning)
- [ ] On chrome, index page images look blurry due to chrome's resize algorithm (as compared to firefox). Consider pre-resizing the images to their native res or displaying them at their native res
- [ ] **Automatically saved output log at end of install includes `output.txt` which may be misleading as this is probably a leftover from the previous install, rather than from when the user ran the game**

Look at the doPost() function for loading feedback, the mod download priority logic, and the UI components for new buttons like 'open downloads folder'. Check how mod sizes are calculated and displayed, and inspect the image rendering on the index page. The output log saving mechanism needs to be examined to avoid including leftover files.

Written by the indexing model from the issue text.