Replace hardcoded entityUris in dialogs with templated links

[carlobeltrame]

In files:

• DialogMaterialItemCreate.vue -> /material-items
• DialogActivityCreate.vue -> /activities
• DialogCategoryCreate.vue -> /categories
• DialogMaterialListCreate.vue -> /material-items
• DialogPeriodCreate.vue -> /periods

The URIs can be retrieved from the API as follows: this.api.href(this.api.get(), 'materialItems').then(uri => { this.entityUri = uri }).
If the root api endpoint does not have some of them already, they can be added in the method apiAction() in IndexController.php.

[feigenmann]

would takle this

[carlobeltrame]

Go for it

[carlobeltrame]

This ticket needs to be re-specified once we are up and running with API Platform. Right now the description is out of date.

[carlobeltrame]

In the meantime, the API is fully-featured again, and this issue can be implemented, based on the issue description.