Improve authors in LaTeX format

[karolina-siemieniuk-morawska]

See https://rt.inspirehep.net/Ticket/Display.html?id=1130365&results=084c372c9cf27d2ede6b0c5b61d97613.

@tsgit mentions:

to me the latex formats on labs look wrong -- why is the author field ending in

.....,author_n and author_{n+1}, \textit{et al.}

why the gratuitous `and` in addition to the et al.?

I don't mind the comma before et al. since it follows a comma separated list of authors

legacy list only one author et al., which is different.

We should improve it

Moved from: https://github.com/inspirehep/inspirehep/issues/1888