Switch from services to engage ui url

[ferishili]

This PR fixes #8 ,

Description

please have a look at the issue's description

What it does

We now use the getOrgEngageUIUrl method from opencast libray base api, which uses the /api/info/organization/properties/engageuiurl in the background.

This means for those institutions that are going to use this plugin while they have multinode opencast setup + lti, they need to do the following:

For old (Upgrades) and new (fresh installs) users:

The Opencast API User must get the role ROLE_UI_EVENTS_EMBEDDING_CODE_VIEW in order to access that endpoint.
Of course it is a default role that Opencast is shipped with, if you have changed the roles of that endpoint in opencast under /etc/opencast/security/mh_default_org.xml you must give that role to the API user!

For old user (Upgrades)

It you are reaching here and it is relavant to you, that means your Opencast API User has already have the role ROLE_GROUP_MH_DEFAULT_ORG_SYSTEM_ADMINS.
Thsi role is no longer needed here, you could remove it from the role list, only if the sole purpose was to use this plugin!

To TEST

It is a very complicated scenario:

• You need to have multi-node opencast (admin-presentation-worker)
• You need to have LTI Auth on you opencast enabled
• You need to have Secure Static File Options enabled in Opencast!
• You need to set the role mentioned above for Opencast API User!
• Configure the plugin properly in terms of current theme in use!
• You need a course with opencast block that hase at least a video ready.
• In that course try to add hvp (Interactive content) activity
• Try to add h5p content into course content bank and use it in the course!
• In every step you have to be able to see the video and its thumbnail!