[JENKINS-74010] Extract JavaScript block in BlueOceanUI/index.jelly

[basil]

Context

See JENKINS-74010.

Problem

blueocean-plugin/blueocean-web/src/main/resources/io/jenkins/blueocean/BlueOceanUI/index.jelly

Lines 71 to 75 in be4d0c9

	<script>
	window.isDevelopmentMode = ${it.developmentMode};
	function lb(c,t){setTimeout(function(){document.getElementById('loadbar').classList.add(c)},t)}
	lb('go',10);lb('long',1000);lb('longer',6000);
	</script>

Solution

https://www.jenkins.io/doc/developer/security/csp/#inline-javascript-blocks

Testing done

Tested together with #2588 and

diff --git a/blueocean-web/src/main/resources/io/jenkins/blueocean/BlueOceanUI/index.jelly b/blueocean-web/src/main/resources/io/jenkins/blueocean/BlueOceanUI/index.jelly
index dc61f3c0d..3d75385e7 100644
--- a/blueocean-web/src/main/resources/io/jenkins/blueocean/BlueOceanUI/index.jelly
+++ b/blueocean-web/src/main/resources/io/jenkins/blueocean/BlueOceanUI/index.jelly
@@ -1,6 +1,7 @@
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:x="jelly:xml">
     <st:contentType value="text/html;charset=UTF-8"/>
+    <st:header name="Content-Security-Policy" value="default-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: ; script-src 'self' 'unsafe-eval' 'report-sample' usage.jenkins.io" />
 
     <!-- Add HTTP headers from extensions. See BluePageDecorator.java -->
     <j:forEach var="pd" items="${it.pageDecorators}">

Note that the above contains unsafe-eval to avoid exposing 74883.

Confirmed that after the header was added but before this PR and #2588, Blue Ocean blew up with a CSP violation. Confirmed that after the header was added and after this PR and #2588, Blue Ocean loaded correctly.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Appropriate unit or acceptance tests or explanation to why this change has no tests
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given