Fix/end date picker validation #870
                
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Proposed Changes
-Fix end date picker so that the initial date is always after the start date and never before it.
-Prevents assertion errors in the date picker dialog when the end date is before or equal to the start date.
-Allows users to select end dates up to 10 years in the future.
Related Issue(s)
If applicable, please link to any related issues (
Closes #123,Closes wger-project/other-repo#123,See also #123, etc.)Please check that the PR fulfills these requirements
(run
dart format .)///).