As an editor I want to see lemma validity indicated in the user interface when editing to avoid data loss and confusing behavior.
**AC**
* Lexeme header in edit mode shows the "publish" button as disabled whilst the information in the form is considered invalid
** redundant spelling variants for multiple lemmas are considered invalid
* Lexeme header in edit mode shows the "publish" button as enabled whilst the information in the form is considered valid and different from the state retrieved at page load
```
GIVEN I am on the Lexeme (header) part of a Lexeme page
AND There exists a lemma with the spelling variant "de"
AND I switch to the edit mode using the edit button
WHEN I add an additional lemma with the spelling variant "de"
THEN The save button is disabled
AND an error is shown saying "It is not possible to enter two lemmas with the same spelling variant"
GIVEN I am on the Lexeme (header) part of a Lexeme page
AND There exists a lemma with the spelling variant "de"
AND I switch to the edit mode using the edit button
WHEN I add an lemma with the spelling variant "en"
THEN The save button is enabled
GIVEN I am on the Lexeme (header) part of a Lexeme page
AND There exists a lemma with the spelling variant "de"
AND I switch to the edit mode using the edit button
WHEN I add an lemma with the spelling variant "en"
AND I click the save button
THEN the lemma is saved
AND if I reload the page the lemma is shown in the lemma section of the page header
```
**Info**
* depends on T196342
* was implemented for form representations in T193636 but implementation of Lexeme header works differently
** produced a mixin (`RedundantLanguageIndicator`) that can be used to determine the validation problem. [[ https://gerrit.wikimedia.org/r/436554 | First steps here ]].