Add methods to expand coverage of `channels` endpoints

[evanofslack]

This PR adds methods and tests to cover the following endpoints:

• [x] channels.create
• [x] channels.invite
• [x] channels.join
• [x] channels.members
• [x] users.info

The method for users.info was required to retrieve user IDs for testing of channels.invite.

Additionally, this PR moves groups methods out of the channels file and into a new file.

[srrathi]

@evanofslack just curious to know is there any specific reason to return slice of User addresses in GetChannelMembers function ?

[evanofslack]

@evanofslack just curious to know is there any specific reason to return slice of User addresses in GetChannelMembers function ?

@srrathi The function's intent is to return a slice of models.User, not user addresses.

func (c *Client) GetChannelMembers(channel *models.Channel) ([]*models.User, error) 

[srrathi]

@evanofslack just curious to know is there any specific reason to return slice of User addresses in GetChannelMembers function ?

@srrathi The function's intent is to return a slice of models.User, not user addresses.

func (c *Client) GetChannelMembers(channel *models.Channel) ([]*models.User, error) 

I'm sorry by addresses I mean Pointer. It is returning a slice of models.User pointer. Is there any specific reason for it.

[evanofslack]

@srrathi Only reasoning is that other methods tend to accept pointers and it is easy to dereference if necessary.

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