Convert the admin and editor UI from Bootstrap 3 to Bootstrap 5.3

[mraible]

Stacked on #155 (2/3 in the stack); review only the top commit.

Converts the admin and editor UI from Bootstrap 3 to Bootstrap 5.3.8. The bootstrap form theme comes through struts2-bootstrap-plugin 6.1.0, which is Jakarta-native and emits Bootstrap 5 markup.

No functionality is removed. Visual and behavioral deltas beyond the restyle, so nothing merges unnoticed:

• Alert banners are dismissible again: the old Bootstrap 3 close-button markup rendered broken under Bootstrap 5, and a .fade { opacity: 0 } override in roller.css (a Bootstrap 3-era leftover) made any fade show component invisible — both fixed, auto-hide after 10 seconds unchanged
• Entries/Comments count badges on the main menu are visible pills again (a bare .badge has no background in Bootstrap 5)
• Themed submit buttons across ~25 JSPs now use the simple theme, so button pairs (Save/Cancel, Upload/Cancel, Yes/No, the entry editor's Save as Draft/Post to Weblog) sit on one row with a primary action instead of stacking in their own form rows
• Field help renders as opt-in Bootstrap tooltips via a tooltip.ftl template override initialised in roller.js; the stylesheet editor's button hints use native title tooltips since the simple theme has no icon slot
• bootstrap-icons replaces glyphicons
• The profile page shows the OIDC issuer and subject as separate read-only fields instead of the raw federated identity string

The result was audited page by page against a seeded master baseline: all 24 admin/editor pages were screenshot-compared with identical data on both sides. That comparison caught two regressions, fixed here: the entry editor's collapsible section headers rendered as blue links, and the date-picker calendar icons in the Entries/Comments sidebars wrapped below their inputs.