Refactor ActionLog block in ExcelPackager to use valuePath instead of column header text.

[jameswsullivan]

While working on the following issues/PRs:

• Last Event/Last Action column not exporting correctly for Excel export #1995
• Event Time column not exporting correctly for Excel export. #2065
• Issue #2065: Event Time column not exporting correctly for Excel export. #2066

William and I discussed about using valuePath rather than column header texts for comparison, but at the time I wasn't familiar with valuePath. I refactored it a bit to use valuePath lastAction -> entry and lastAction -> actionDate now, which should be more reliable than using column header texts.