Resolve "Version dropdown goes wrong if versions are not monotonic"

What does this MR do?

Fixes bug raised in #13226 (closed).

Bug stemmed from the fact that, previously, the version id (obtained from the query param) was being compared to allVersions.length, when instead it should be compared to currentVersionNumber: version id could be anything, whereas `0 < currentVersionNumber < allVersions. See example scenario below.

Relevant tests have also been updated to reflect the new logic.

Additional changes:

• improved test coverage
• converted some methods to computed values to follow best practice

Example scenario where bug could have previously occurred

If we had an issue with 5 designs, with ID's [1, 2, 3, 5, 6], and selected the 4th design (ID = 5), then the UI would indicate it to be the latest design (because its ID happens to be the same as the number of designs we have). This is incorrect; the latest design is 6.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers