[WIP] EZP-29279: Values of Date Field Type should be handled in UTC only

[jacek-foremski]

JIRA issue: EZP-29279

Currently, in Legacy the timestamp stored in the database is always midnight in the server's timezone, while in Platform it is always in UTC. This PR changes how values of Date Field Type are handled to make the behavior consistent with Platform. It also makes sure that the displayed values are also in UTC and unaffected by server's timezone.

TODO:

• Migration script that changes values saved in the database to UTC.

[andrerom]

Another todo here:

• Check if only eZ Date datatype uses this lib class, as the fix seems to be in the wrong place on first view but if only used for Date datatype it can be ok perhaps.

[mateuszbieniek]

@andrerom I agree - eZDate is used in other places, so I prepared new PR with a different approach:
#1401

[andrerom]

Closing in favour of #1401 as this PR is too intrusive on lib classes used several places in the system.