[FIX] web: multiple opened modal need fixing since bootstrap lib update
[FIX] website: slug: fallback to positive ID when slug appears to contain a missing negative ID

In some rare cases database records have negative IDs,
so the slug URL could look like /foo--20. This could
be mistaken for a slug ending with a `-` and a positive ID.
The latter is not supposed to happned as final hyphens
are stripped by slugify, but has been used in the past
and may be used in old links.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64