Fix Autolink admin config parsing

[hanzei]

Summary

The plugin was falsely storing it's config in UperCase names, which lead to the webapp not being able to rea

This PR make use of json.Marshal to convert the plugin config to an map[string]Interface{} and hence json tags are used everywhere now.

There is one migration left. If an admin (after updating to this PR) sees the empty list in the system console and inputs something, the existing admin list will be overwritten. Running /autolink add or /autolink delete brings the configuration in the right state again.

Ticket Link

#180
#181

[hanzei]

@dipak-demansol gentle reminder to review this PR

[dipak-demansol]

@dipak-demansol gentle reminder to review this PR

@hanzei @DHaussermann i'm not able to regenerate the 180 & 181 issue with the master Build. Its working fine with the master build.

[hanzei]

@dipak-demansol Did you test with a clean plugin config i.e. PluginSettings.Plugins.mattermost-autolink is empty?

[dipak-demansol]

@dipak-demansol Did you test with a clean plugin config i.e. PluginSettings.Plugins.mattermost-autolink is empty?

Yes

[hanzei]

@dipak-demansol Could you please test with the following steps:

1. Set an autolink admin via the system console
2. Run /autolink add foo to create an link
3. Check the list of autolink admins in the system console

Observed behaviour on master:
The list is empty

Expected behaviour:
The list contains the admins set in step 1

[dipak-demansol]

@hanzei tested as you told and its LGTM, adding the cases with that i tested, pls let me know if i miss any case to test.

1. adding two member-Users id in console,those 2 user is able to use autolink slash command.
2. after performing First case, in the admin system console i refresh the page and still all data is there and those Two users are able to use to autolink slash command. (its a case that ben told me to check) (issue 181)
3. by adding space between the two id, expected result is slash command should not work for both user.:- as expected.
4. by adding space before the first id, still both user is able to use the slash command and i don't have any issue with that so it LGTM.
5. by adding space before the first id and adding space after the last id, both User are able to use the slash command.
6. added the user-id on the config file and restarted the server then i'm able to see that config value in the system-console. (issue 180)