Use @cache_control_expires to fix bug 1004917

Replace @cache_page on release_notes and system_requirements
[fix bug 1011476] When closing Tabzilla return focus to tab when pressing Esc.
Merge pull request #2018 from jgmize/relnotes-cache_control_expires

Use @cache_control_expires to fix bug 1004917
Merge pull request #2019 from alexgibson/bug-1011476-tabzilla-return-focus-to-tab-esc

[fix bug 1011476] When closing Tabzilla return focus to tab when pressing Esc