New ESLint plugins to enforce DSpace-specific coding conventions

[ybnd]

Before committing to #2865 we'd already worked on a few ESLint rules for some common situations in DSpace:

• Enforcing aliased imports like import { of as observableOf } from 'rxjs';
• Making sure entry component decorators in themes actually include the theme name
• Making sure entry component decorators are not duplicated
• Enforcing that ThemedComponent wrappers do not set default values on inputs (see Fixed some themeable component default values not being overridable #2164)

We have prototypes for these four rules, and that we'd like to polish up and integrate into DSpace Angular's custom ESLint plugin