Fix broken links in HTML

[gmantele]

Update LaTeX macros referencing IVOA docs and remove xspace package dependency

xspace was used in these LaTeX macros. Due to it, links to IVOA documents were sometimes broken in the HTML output.

This commit is a cherry-pick from db4a0fc08fa4892ba9e6e5c644451b088a358a8d

The issue was already discussed by email in January 2018: http://mail.ivoa.net/pipermail/dal/2018-January/007942.html.

Fixes #60

[gmantele]

I checked both the PDF and HTML version. It seems to be ok now.

I am just not sure about the removed \xspace in macros like \SectionRef & co. It does not seem to change anything in the output. Most of the time a space was already manually appended when needed in ADQL.tex. So, I think it should be fine. But tell me if I am wrong.

[gmantele]

I note you used a force push early in this PR - things seem intact but I'm always wary of force push as I have seen it create huge issues in repos in the past.

I rarely use it because of the issues you're speaking about. The only times I use it is when I am alone on the repo (or here on a branch) and when I performed a rebase (here, it was the case). But yes, this is something I usually prefer to avoid. In this particular case, I did not want to spoile the commit history with additionnal merge commits for...basically nothing (a rebase with no real conflict).