Add one optional `communication channel` field in Grant

### User Story

[comment]: # (As a , I want to so that .)

The user story is requested by some funders of Grants who want to find communication channels such as Discord channel, Telegram group, or gitter to ask questions or discuss with the project owners and community.

As a funder, I want to find a link to join one communication channel such as Discord, Telegram group or gitter, and discuss with the project owners and communities, so I can learn more about the projects in details before I fund the grant.

### Why Is this Needed

[comment]: # (Describe the problem and why this task is needed. Provide description of the current state, what you would like to happen, and what actually happen)
*Summary*: the feature makes it easy for funders to engage with the project team, and also help the project teams to bring more users or stakeholders into their communities.

### Description

[comment]: # (Feature or Bug? i.e Type: Bug)
*Type*: Feature

### Current Behavior
[comment]: # (Describe what actually happened.)

It's hard for a funder to find a communicatoin channel such as Telegram, Discord or gitter of the project.

### Expected Behavior
[comment]: # (Describe what you expected to happen.)

An optional field of communication channel can be found on the Grant page, for users or funder to join the grant community by a single click.

### Definition of Done
[comment]: # (Any other information that would be useful, bullets are helpful.)

A `Communication Channel` field is added in the **Create Grant** page: https://gitcoin.co/grants/new

### Data Requirements

the # of clicks of the communication channel link

### Additional Information
[comment]: # (Any other information that would be useful, content, screenshots, etc.)

The feature request comes from discussion in one meetup of the Gitcoin Chinese community on July 1st, 2021