Refactor loadModelRuns to the store as a global action

[floryst]

• loadModelRuns is now part of the store as queryModelRuns and updates store state.
• queryModelRuns manages pagination state through "firstPage" and "nextPage" requests (see ModelRunList.vue for usage).
• checkDownloading is now part of the store as refreshModelRunDownloadingStatuses. Additionally, it has been fixed to properly update the downloading statuses of all loaded pagination pages.
  • A side note: I have yet to test this function!
• All usages of state.downloadCheck have been replaced with calls to refreshModelRunDownloadingStatuses.

[floryst]

This technically conflicts with #502, but it's still reviewable in its current form. This should be merged into/after #502.