deps: react-bootstrap 0.31 -> 2 (Bootstrap 5 CSS deferred as debt)

Removes the largest batch of React-19-blocking warnings: the 0.31
components (Grid, FormGroup, ControlLabel, Navbar.Header/Brand, Checkbox,
SafeAnchor) all leaned on legacy context / defaultProps.

29 files converted: Grid->Container, FormGroup/ControlLabel/FormControl/
HelpBlock -> Form.Group/Label/Control/Text, Checkbox -> Form.Check (label
as prop), bsStyle->variant (default->secondary), bsSize->size,
pull-right->float-end. Custom Col.js now re-exports v2's Col; custom
NavItem.js rewritten self-contained (no SafeAnchor/createChainedFunction);
Navigation.js drops react-bootstrap Navbar (raw markup anyway).

CSS stays on Bootstrap 4 (alexwine:bootstrap-4) for now: the BS4->5 jump
is entangled with the jQuery carousel/swipe + navbar-collapse and is
tracked as separate debt in UPGRADE.md. Only .form-label needed a shim
(forms.scss). Browser-verified home carousel+navbar, signup form + terms
checkbox, login form; REST smoke byte-identical.

imports/ui/components/SelectionMap/SelectionMap.js

@@ -241,7 +241,7 @@ class SelectionMap extends Component {
                   <ButtonGroup>
                     { this.props.sndBtn && this.props.onSndBtn &&
                       <Button
-                          bsStyle="warning"
+                          variant="warning"
                           title={this.props.sndBtnTitle}
                           onClick={event => this.onSndBtn(event)}
                       >
@@ -249,7 +249,7 @@ class SelectionMap extends Component {
                       </Button>
                     }
                       <Button
-                          bsStyle="success"
+                          variant="success"
                           disabled={this.props.disableFstBtn}
                           onClick={event => this.onFstBtn(event)}
                       >