Determine how contextual attributes should be populated.

• Determine how contextual attributes should be populated. The contextual attribute dropdown list show the attributes according to the schema type that was selected by the user
  • I have included all the content we had in ContextualAttributes.vue as part of Schema.vue. They are fully related and also, that way, we have the opportunity to disable contextual attributes dropdown list until users choose any schema type (web, app, custom). Otherwise that field will be set as readonly
  • I have also removed agent_app_install_id from sample database because that field is also added automatically when appropriate. Users will never have the opportunity to add it themselves so I think it doesn't make sense to use it as a sample value
  • For now, when custom schema type is selected, all contextual attributes are available to be selected
  • If users change the schema type, the selected contextual attributes will be reset

Also taking the opportunity to make a couple of minor changes:

• New CdxIconPower icon instead of the cdxIconAlert we had (postponed while working on T377819 because the icon wasn't available) (T380230)
  • New codex version (1.17.0). It was needed to be able to use the new icon mentioned above
• Description is now an optional field (T377353)

Closes T366951, T377353, T380230