Adds a mock-mode spec that captures the workspace overview, issues tab,
issue detail, and PR detail states for PR screenshots. Mock event ids are
now full 64-hex so share-link copy buttons render in mock mode, and
post-screenshots.sh no longer needs bash 4 (mapfile/declare -A) so it
runs on stock macOS bash 3.2.
Signed-off-by: Thomas Petersen <thomasp@squareup.com>