Issue 936 - standardize lesson submissions

[acrymble]

This pull request closes #936 and adds a Proposal Form to the 'authors guidelines' page that encourages authors to use our new form.

@jenniferisasi can you please upload the Spanish lesson template form to the /assets/forms/ folder on branch Issue-936, and adapt the first sentence of /guia-para-autores so that people can find it? You said you updated the form but I don't have a copy so I can't do it myself.

The sentence on the English page reads: "If you have an idea for a new lesson, or have already written a tutorial that you think could be adapted for the Programming Historian, complete a lesson proposal form and contact..." and is linked on "proposal form".

@mdlincoln I've uploaded the French form, but the French author's guidelines file isn't online yet, so I haven't been able to change it so that people can find this new form.

[mdlincoln]

@acrymble you will find the French guidelines file on the french-staging branch. If you can add it there now, then it'll publish when we eventually merge in that PR.

[mdlincoln]

Update from 2018-11-29 meeting, remaining tasks are:

• @jenniferisasi has a translation of the proposal form in Spanish. She will send to @acrymble to add to this PR.
• Link on the Spanish author guidelines to the Spanish proposal form

After that is done, we can merge this ticket. @mdlincoln will add the appropriate links for French himself.

[jenniferisasi]

I think we can close this issue?