v2.19: A second full test run

Fixes and touches from using Sprintwise start to finish again, plus speed-ups for teams with many sprints.

• Jira issues with very long descriptions are cut to fit instead of being skipped, so Sync keeps their status up to date. They aren't sent back, so Jira keeps the full text.
• Story points accept a decimal comma: 1,5 is 1.5.
• Point totals never show long decimals such as 0.30000000000000004.
• A new story never reuses the key of a deleted one that's still in a sprint.
• An edit saved while Jira syncs, or while stories are being sent, is never overwritten or lost.
• Issues created in Jira between Preview and Import are picked up by the next sync.
• Two snapshots saved at the same moment, from two tabs, can't double the change log.
• Sub-tasks are left unticked at import, like epics, and new ones aren't added by Sync.
• The backlog says why a story scoring 80 or more isn't Ready, such as "Too big".
• Running sprints show which day they're on and how many are left; a new sprint suggests its name and dates.
• Refinement leaves out stories the current sprint is already working on.
• A running sprint's report reads "so far" rather than as a final result.
• Shared reports and backlogs say what made them, for viewers who'd like the same.
• The Sprints page and sprint page load less, and two database indexes keep edits quick as history grows.