Force uniqueness of IDGLOSS field when creating a new sign entry

[stevecassidy]

Originally reported by: Steve Cassidy (Bitbucket: stevecassidy, GitHub: stevecassidy)

---

Currently IDGLOSS can be shared between signs but this causes problems when trying to view a gloss because the code assumes that it is unique.

We should ensure that IDGLOSS is unique when creating a sign. Not sure if this should be a database level unique constraint - is the BSL practice different to this?

---

• Bitbucket: https://bitbucket.org/stevecassidy/signbank/issue/99