Allow internationalization for 'Posts'

[latiif]

Prerequisites

• This pull request fixes a bug.
• This pull request adds a feature.
• This pull request introduces breaking change.

Description

Allows for customization in the internationalization files for "Posts".

If there is an entry in the i18n/ files under 'Posts' (Uppercase P), it will be used in the posts page.
If not, it will revert to what would've been displayed instead, and not create a breaking change.

NOTE While this is NOT a breaking change, we still need to update the i18n/ files for the changes to reflect.

Issues Resolved

How to change /posts page title? #70
i18n translation used in layout files #237

[luizdepra]

I think is better to do something like this {{ title (i18n (lower .Title)) | default .Title }}. So, you should put "posts" as id in the language file. Yep, lower case.
Also, you need to translate the title inside the other define. This is the title shown in the browser tab/topbar.

[latiif]

I tried to make it more DRY by using a variable or a function definition. But I could not, Golang templates does not seem to allow global variables. Are you aware of a work-around?

[luizdepra]

Nothing that I can remember, @latiif. But there is no problem with little repetitions in this case.

Thank you.

[a1x42]

@latiif other translations are capitalised when displayed e.g. my translation file contains téma for tag but on the list page for a specific tag it says Téma. For the posts translation this does not happen. could this be made consistent please?

[latiif]

You are correct. This was an oversight as I needed/tested this for Arabic which does not have lowercase/uppercase. A fix is suggested in #338.