Implementation of the /api/days method, some documentation updates

[kolayne]

Relates to #19

So sorry I make backwards-incompatible updates of the docs, while front-end is being built on top(?) of them! :'(
Apparently, I didn't think deep enough when drawing the documentation up. There are mostly clarifications or new details added, but there are currently at least two incompatible fixes: I replaced the 400 Bad Request status code with 412 Precondition Failed for some cases and updated date format for one method. So, @tanya-kta and anyone else who needs the documentation, please, take a look at the changes and tell me if there is something unclear about what I've added (it's a bit late now, so I might have written some nonsense or explained something too complicatedly)

Welcome the new method for adding new days!

Is blocked by #29 because of the following:

life_calendar/src/webhandlers.go

Lines 118 to 120 in b31770c

	session, _ := cookieStorage.Get(r, "session")
	// Panics if user is not authorized. Will be fixed with the appropriate middleware
	userID := session.Values["id"].(int)

[kolayne]

@DarkSquirrelComes, I am planning to request your complete review on the PR when I finish it, but I have a question so far. I hate how these lines look together (especially if I remove the blank line). Is there a nice way to fix it? Our hard limit is 120 characters per line (including the indentation, my tab size is 4)

I would also be thankful if you have checked other TODOs added in this PR and tell me if there is something you disagree with or know a better way for. Thank you!

[DarkSquirrelComes]

This autoformat seems good-looking to me. But I don't really understand, why you compare lens of this values?

[kolayne]

This autoformat seems good-looking to me.

Then should I level the empty line there? Look what happens if I remove it:

	// TODO: the two lines below are formatted automatically this way. I wonder if it is possible to write it better
	if len(r.Form["activity_type"]) != len(r.Form["activity_proportion"]) ||
		len(r.Form["emotion_type"]) != len(r.Form["emotion_proportion"]) {
		js, err := json.Marshal(map[string]interface{}{"ok": false,

I can't understand where the condition of this if ends and the body begins based on the indentation!

But I don't really understand, why you compare lens of this values?

Each activity/emotion here is described with one type and one proportion. I ensure there exists a type for each proportion and a proportion for each value. You can see more details on this method in the documentation

[kolayne]

That's pretty much it. Waiting for @VladimirKoryakin to resolve #29.

I would also like the reviewers (@DarkSquirrelComes, @tanya-kta) to pay additional attention to comparing the documented and the actual behaviours of the method in this PR! Please, ensure all the errors expected to be returned are returned in all the appropriate cases

[kolayne]

Note: I want both two reviews on this

[kolayne]

One more thing: the database schema is invalid because PostgreSQL does not support CHECK constraints that reference data that is not in the row being inserted or updated (docs). This must be fixed before the PR can be merged (if this will ever happen)