Fix warnings in frontend tests

[BacLuc]

Invitation.vue: do not use optional chaining in template

The transformation jest uses cannot collect coverage for optional chaining (?.).
Failed to collect coverage from /home/runner/work/ecamp3/ecamp3/frontend/src/views/camp/Invitation.vue
ERROR: Unexpected token (1:1001)
STACK: SyntaxError: Unexpected token (1:1001)
    at Parser.pp$4.raise (/home/runner/work/ecamp3/ecamp3/frontend/node_modules/vue-template-es2015-compiler/buble.js:2757:13)
    at Parser.pp.unexpected (/home/runner/work/ecamp3/ecamp3/frontend/node_modules/vue-template-es2015-compiler/buble.js:647:8)
    at Parser.pp$3.parseExprAtom (/home/runner/work/ecamp3/ecamp3/frontend/node_modules/vue-template-es2015-compiler/buble.js:2196:10)

frontend: transform package url-template in jest tests

Else jest complains:
Jest encountered an unexpected token
...
/home/runner/work/ecamp3/ecamp3/frontend/node_modules/url-template/lib/url-template.js:97
      export function parseTemplate(template) {
      ^^^^^^

Admin.vue: register CampConditionalFields component

Else jest complains that the component is not registered.
Also fix Admin.spec.js, was broken in 020ffab.

[usu]

Seems the problem with optional chaining was fixed with jest 29:
vuejs/vue-jest#200 (comment)

So maybe it would still be worth to try merging #3566

What do you think?

[BacLuc]

Seems the problem with optional chaining was fixed with jest 29: vuejs/vue-jest#200 (comment)

So maybe it would still be worth to try merging #3566

What do you think?

Until now we had only 2 places were this optional chaining was used.
And it can still be used, just in computed properties.
Here it was even not needed, because the nullable parts were anyway checked by v-if before
When we anyway have to switch to vue3 in hopefully less than a year, i would say we save us the problems with additional hacks, and
use the new features when we have a dependency tree where we receive updates.
(See the blocked vue dependencies in #1864 )

Another way with which we can get away from the jest problems is switching to vitest (#3268 )

[BacLuc]

converted it to draft, because this work may be unnecessary with #3268

[BacLuc]

not needed anymore with #3268