Fixed issue #7224: comparison of dates (partial fix)

Dev: LEMval() now returns answers to date questions in yyyy-mm-dd HH:MM
Dev: format, independent of the date format that is chosen for the
question
Dev: or the particular survey. Thus, comparisons between different date
Dev: questions are possible (e.g. relevance or validation equations
Dev: "arrivaldate > departuredate") even when one question is in
Dev: dd.mm.yy and the other in mm/dd/yyyy HH:MM format
Dev: (same or previous page).
Dev: qanda_helper.php: min/max logic had to be changed accordingly
Dev: and is less complicated now.
New feature #8524: implements min/max date in EM

Dev: implements min/max date in EM (so dates are validated and
Dev: the correct help messages appear). Adv. attributes min/maxdate
Dev: in date/time question supports yyyy, yyyy-mm-dd,
Dev: questioncode (another date question) and expressions
Dev: (eg. date(Y-m-d H:i, strtotime(birthdate)+60*60*24*365*18).
Fixed issue #8484: expressions in min/max date referring to questions on same page

Dev: Expressions in adv. attribute min/maxdate also work for the date
picker
Dev: if variables in the expression refer to questions on the same page.
Dev: better hiding of a hidden element (min/max date)

Dev: Thank Shnoulle!
Dev: date format in help message now matches question date format
Fixed issue #8531: Wrong date format for [code].shown (EM)

Dev: .shown used to work only if referred date question
Dev: was on the same page. Now also on following pages.
Fixed issue #8510: Survey import/copy not working
Dev This seems to be a PHP bug as I could find no logical reason that it crashes out with duplicate database entries.  For some reason the foreach loop gets messed up and the pointer is reset in the midst of the foreach - caused by the database insert. Makes no sense at all and does not happen on newer PHP versions.
Fixed issue #8497: Long email address fields are truncated when utilizing "Add participants to central database" from token table
Fixed issue: File download not working if one of the files in the response is missing
Fixed issue #8532: Check for minimum memory available fails if PHP has "memory_limit" equal to -1
Dev: fix javascript error when token editing
Fixed issue #8488: Language display instead of abbreviation in edited records in token list
Update caption_view.php

Send button never get enabled, because it's a jquery-ui button
Merge pull request #168 from s4mdf0o1/patch-2

Update caption_view.php
Fixed issue #8541: str_replace seems to be replacing just the first occurrence of search string rather than all occurrences
dev: added admin footer to plugin layout (as a widget)
dev: made plugins / pluginsettings use the same rendering using bootstrap in the right order
Fixed #8466: CSS/HTML issue with Plugin/settings
Fixed issue #08538: Problem with array (numbers) questions error information
Dev: Fix multi javascript testing for error/good
Fixed issue #8489: Non-operational CAPTCHA due backup to resume later.