add validation rules to editors

[kkrebs]

We need more validation rules to prevent:

• number twist in date fields: 1986-31-12 instead of 1986-12-31
• strings in date fields like '-'
• invalid string as link to other mcrobjects instead of object ID

[annplaksin]

• References from expressions to persons (composer & lyricist) should not be mandatory.

• References from sources to persons (e.g. editor, authors, funders etc., responsible persons for scribe hands, involved persons for events) should not be mandatory

• Every expression must be referenced in a source via a source component

• A work needs at least one expression

[sebhofmann]

see also #160 (comment)

[ja-roh]

I could enter "strange" dates. There was no error message, but the date was not show in the final entry. Thus, I believe some aspects of validation may not be working properly at the moment.