Adding a HttpUrl Validator to vendor SimpleValidator
I made up my mind, will change the layout to fit better into the MVC pattern and avoid to many stupid data lookups at wrong places.

Now there is a user attribute 'icon_url', which is stored in the DB (only SQLite for now, MySQL/Postgres still needed!).

This is backup with a 'icon_url_default' setting in the application config, which will ensure that there are icons for people without
data on Gravatar or no email address set. This setting will also be used to check if we should disable avatar icons and perhaps maybe
later extended to fetch data from external sources like LDAP or local storage. (Maybe LDAP jpegPhoto can be imported to local storage
on creation of new user?) It is allowed to have an empty default set, which will be resulting in a transparent <img> or no <img> element
included in the HTML code.

'form_select' in helpers is changed to use an array $data_img which will include all the data for imagepicker if this should be used.
It is allowed to use an empty URL which will be silently replaced with a transparent picture.
Removed no longer valid class parameter from the assignee <select>, this needs to be replaced with an array provided from the Controller
Changed Config Model to use a const for the email replace pattern, so we can reuse this in helpers.
Removed function get_user_gravatar from helpers; Changed get_user_email to get_user_icon_url; Added new function inline_avatar to helpers, generating the HTML; changed the app.css necessary for the renaming and styling