Fix trailing slash for alt languages on home page

[nbusseneau]

I think this should fix #54.

[nbusseneau]

Got the idea after updating to v1.4.2 and looking at changes in #57 / #58.

[nbusseneau]

@rhukster Revisiting old PRs -- not sure where #54 went, seems like it disappeared in a black hole, however the gist of it is that alternate languages are linked with a trailing slash on the home page, leading to:

• Inconsistent navigation links based on current language (current language is non-slashed, other languages are slashed).
• "Trailing slash jumping" between the slashed and non-slashed URL (as you click on the slashed URL but it gets unslashed by Grav routing).

Akin to #58, we check if the route is exactly / and if yes simply discard it.