1.6.x bakcport of #1188: Adds posibility to set the background image for the About modal

[mmelko]

Backport of #1188

[tadayosi]

@mmelko Pls don't forget to send pull req to website project.

[github-actions]

Test Results

  8 files  ±0    8 suites  ±0   57m 23s ⏱️ + 10m 36s
 66 tests ±0   64 ✅ ±0   2 💤 ±0  0 ❌ ±0 
536 runs  ±0  508 ✅ ±0  28 💤 ±0  0 ❌ ±0 

Results for commit 2de27d3. ± Comparison against base commit 8e5f3b7.

[hawtio-ci]

Test results

Run attempt: 2604
Detailed summary

NAME	TESTS	PASSED ✅	SKIPPED 💤	FAILED ❌	ERRORS 🚫	TIME 🕖
test-results-quarkus-node(18)-java(17)-firefox	67	64	3	0	0	424.475
test-results-quarkus-node(18)-java(21)-firefox	67	64	3	0	0	432.133
test-results-quarkus-node(20)-java(17)-firefox	67	64	3	0	0	424.306
test-results-quarkus-node(20)-java(21)-firefox	67	64	3	0	0	424.885
test-results-springboot-node(18)-java(17)-firefox	67	63	4	0	0	441.038
test-results-springboot-node(18)-java(21)-firefox	67	63	4	0	0	427.432
test-results-springboot-node(20)-java(17)-firefox	67	63	4	0	0	444.53
test-results-springboot-node(20)-java(21)-firefox	67	63	4	0	0	424.755