Base setup for i18n for server with go-i18n library

[Kshitij-Katiyar]

Summary

It includes:

1. Added the go-i18n library v2.
2. Added sample translation and also en.json file

Fixes https://github.com/mattermost/mattermost-plugin-github/issues/783

[mickmister]

In order to include localization in templates, we'll need to have the code-operated parts (i.e. markdown link construction) of the message be parameterized into the translation string. We can pass the calculated sub-template calls as a dictionary argument to the localize function

With this in i18n/en.json

{
     "newPullRequestCollapsed": "{{.RepoName}} New pull request {{.PullRequestTitle}} was opened by {{.UserName}}.",
}

func localize(bundle *i18n.Bundle, lang string) func(id string, data map[string]interface{}) string {
    return func(id string, data map[string]interface{}) string {
        localizer := i18n.NewLocalizer(bundle, lang)
        return localizer.MustLocalize(&i18n.LocalizeConfig{
            MessageID:    id,
            TemplateData: data,
        })
    }
}

---

Then we can change

https://github.com/mattermost/mattermost-plugin-github/blob/e958064b9d26bd23cee76d385e721c7b4e654fee/server/plugin/template.go#L207

to

{{ localize "newPullRequestCollapsed" dict "RepoName" (template "repo" .Event.GetRepo) "PullRequestTitle" (template "pullRequest" .Event.GetPullRequest) "UserName" (template "user" .Event.GetSender) }}

---

Or we could instead make local variables in the template like this for potentially better readability, but we'll need to implement an expose a templateToString function to return a string:

{{ $repo := templateToString "repo" .Event.GetRepo }}
{{ $pullRequest := templateToString "pullRequest" .Event.GetPullRequest }}
{{ $user := templateToString "user" .Event.GetSender }}
{{ localize "newPullRequestCollapsed" dict "RepoName" $repo "PullRequestTitle" $pullRequest "UserName" $user }}

[raghavaggarwal2308]

@wiggin77 SUMMARY: We did the base setup to support translations in this PR, but we were facing some issues while supporting translations in a template file. We tried multiple things for the same as suggested by @mickmister , but were not able to achieve it completely. For basic text inside the template we were able to achieve it and the code is present on this https://github.com/mattermost/mattermost-plugin-github/tree/template_i18n_translations branch.

The only thing that we were not able to support is where we are calling an internal function inside a template