Change default dashboard name to overview

What does this MR do?

Related issue: #225168 (closed)

Changes all references from "default dashboard" to "overview".

The main goal of this MR is the user-facing change of terminology from "Default dashboard" to "Overview". However, there being lots of references in the code to "default dashboard", in order to use the same language as our users, changes in code to those references are also needed.

The most important, user-facing reference is in the dashboards dropdown:

Before	After

Screen_Recording_2020-07-30_at_16.49.44

Other references should be changed in:

• FE code
• FE tests
• BE code
• BE tests
• docs
• translation files

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team