Switch back from Roboto to Helvetica on bootstrap pages

I searched on loomio and apart in the [discussion about the branding](https://www.loomio.org/d/zsxwVAyP/change-our-branding-to-diaspora-lower-case) (where we discussed about the lower case but never really about the font) I didn't find why we switched from Helvetica to Roboto for the bootstrap pages.

The feeling of inconsistency I have when I compare bootstrap pages and other pages mostly come from that. We can discuss which font we prefer, but during the discussion, I really think that diaspora* should use one and only one font. As most of the pages are currently not ported to bootstrap, I open this pull request which restores Helvetica as the default font.

There is still some part of the code where Roboto is used, I can change them here too if we want.