[NEXUS-6189] take-3, override sendChallenge and if the request looks like its make from a browser, render an error page, otherwise perform default (send www-authenticate).  This makes the omission filter use-less so removing it.

Conflicts:
	components/nexus-core/src/test/java/org/sonatype/nexus/web/internal/BrowserDetectorTest.java
	components/nexus-webapp/src/main/java/org/sonatype/nexus/webapp/WebappModule.java
port 401 error page handling
Merge pull request #297 from sonatype/NEXUS-6189-expose-global-session-timeout

WIP: [ENXUS-6189] expose shiro.globalSessionTimeout configuration
Merge pull request #310 from sonatype/backport-2.7-send-error-page-401-from-browser-request

REVIEW (2.7) [NEXUS-6189] Backport 2.7 send error page 401 from browser request